When you troubleshoot modem connections, it is important to evaluate
three main areas: the client modem, the telco network and the Network Access
Server (NAS). You must ensure that the modems on either side, and the telco
network work properly, because problems with any of those factors can affect
modem connectivity. This document discusses how to optimize and troubleshoot
client modem related issues.

This document is not restricted to specific software and hardware
versions.

The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.

After you examine the server side of the connection, look at the modem
or connection of the client as a possible cause of difficulty. In many cases, a
particular client modem on a particular phone line can have problems when it is
connected to a digital modem. Problems can include failure to train up, or a
successful train-up but poor throughput or premature disconnection.

Based on the assumption that the phone circuit works, and holds the
server modem constant, you can fix the problem with the client modem. To do so,
you must improve the code of the client modem, or reconfigure the client
modem.

First, determine what kind of modem the client uses. Find out the
vendor, the hardware model, the software (firmware) version, and the underlying
modem chipset. Although there are hundreds of different modem vendors, there
are only a dozen or so different chipsets.

Details about the modem vendor and hardware model must appear on the
modem and its packaging. For modems sold in the United States, see the FCC ID
and Part 68 registration number found on the modem packaging. Refer to the
FCC ID
Search Help Page for more information.

To determine which chipset you have, use the
ATI commands. Use HyperTerminal to connect to the
COM port of the modem, and issue the ATI0 through
ATI11 commands. Some of these commands can yield
errors, but they usually provide enough information for an educated guess.

If a client modem has performance problems, it is useful to configure
it to use slower modulations, or to use slower rates within the chosen
modulation.

For example:

Problem 1: A client connects in V.90 at 49333, but
suffers from a premature disconnect after two minutes.

Solution 1: Configure the client to cap its V.90
receive speed at a lower speed (for example, 44000). If the client still
suffers from erratic performance, configure it to use V.34 or below.

Problem 2: A client attempts to train up in V.90,
but fails to train up at all.

Solution 2: Configure the client to use K56Flex,
V.34 or below. If the train up still fails, configure the client to connect at
a maximum V.34 rate of 21600. If that still fails, configure the client to use
only V.32bis or below. If that fails, configure the client to use only V.22bis
or below. If that fails, get a new modem and phone
line.

The modem vendor must supply documentation with the modem. If this is
not available, refer to these links for more information:

While you try to reconfigure the client modem, use HyperTerminal (or
some other terminal program) instead of Windows Dial Up Networking (DUN). This
is because, DUN does not generally display details of the call placed.

If necessary, temporarily reconfigure the NAS lines to permit
terminal dialin. That is, if the async interfaces have async mode
dedicated configured, change them to async mode
interactive, and put autoselect ppp on
the lines. If you use Authentication, Authorization, and Accounting (AAA), the
AAA server can require adjustment to allow interactive
logins.

On the client PC, start the terminal program. If you use
HyperTerminal, create a new connection. Create a name and icon. In the
Connect To panel, select Connect using COM
port.

In this case, the COM port refers to the COM port on which the modem
is. In the COM Properties panel, set 115200 bps, 8
databits, no parity, 1 stopbit, flowcontrol hardware (see the
Example HyperTerminal Session section for
details). Enter the AT command, and ensure that an
OK response appears. If no response appears, there could be a
cable problem or, on an older modem, the speed has to be slower in the COM
properties.

Reset the modem to the factory defaults (use the hardware flow
control template, if it is available). This will typically be
AT&F or AT&F1. If the factory default
does not use these settings, set the modem to provide data communications
equipment (DCE) rate information at connect time (typically
TW2), and ensure that the speaker is on, until carrier detect
(typically ATM1).

To establish a performance baseline, dial manually into the NAS with
the ATDTnnnnnnn command. See the AS5x00 Case Study
in the Verifying
Modem Performance documentation for an example.

Here is the output from a connection to one of Cisco's test systems.
First, enable the speaker and DCE rate information reporting:

atw2m1
ERROR

As it turns out, w2 is not necessary on US Robotics
modems.

atm1
OK

Next, dial into a static lab:

at
OK
atdt914085703932
NO CARRIER

The normal connection seems to fail. In this case, it is a noisy
line, so set the modem to factory defaults (&f), turn on
the speaker (m1), and cap the modem at 28.8
(&n14) with the at&fm1&n14
command:

You have determined that the connection seems to work with the new
settings. Now you must update the configuration of the modem for dialup
networking to reflect the changes.

To help troubleshoot modem problems, configure Windows to create a
modemlog (\windows\modemlog.txt). The configuration setting only works with
Telephone Application Programmable Interface (TAPI)-enabled programs, such as,
DUN and HyperTerminal.

Complete these steps to set up modem logging or customer parameters on
a Windows 95/98 system:

If any additional settings are needed for the modem connection to
succeed, enter those commands in the Extra settings text box.
Based on the previous example from the Example
HyperTerminal Session section, the &n14
command has been added.

Figure 6 – Specify Extra Settings and Enable Modem
Logging

Click OK.

The procedure for modem logging and custom settings for Windows NT
4.0 comprises similar steps. The file will be called
modemlog_modemname.txt. The file appears in the system
root directory (which is usually the winnt directory, unless
set otherwise). Modify the Registry Editor to enable modem logging and settings
on Windows NT 3.x.

When you attempt to diagnose premature disconnect problems on a PC, it
is helpful to know the type of information that was sent over the link at the
PPP level. Windows 95/98 is able to create a PPP log file whenever the PPP
adapter is used (/windows/ppplog.txt).